It is generally well known that electric trolley buses are powered by a pair of overhead wires which carry an electric current. The wires are aligned in parallel over the main lane of traffic at a generally uniform height over the traffic lane, for the length of the intended bus route. The electric current is transmitted into the bus through a pair of collectors, one each attached at the upper end of a pole pivotally mounted on the bus roof, each of which is spring biased to keep the respective collector pressed upwardly against the proper overhead electric wire. Such a collector, more commonly known in the transit industry as a "harp", includes an electrical contactor mounted onto a ball-and-socket swivel joint, with the electric contactor at an uppermost position to engage the overhead wire. The swivel joint mounting permits the bus to be free to move laterally, from curb to driving lane or lanes; i.e., to permit "touring", and yet allow the electrical contactor, more commonly known in the transit industry as a "shoe", to maintain its engagement with the overhead wire. The ball-and-socket swivel joint not only allows lateral movement of the bus with respect to the overhead wire, but will also compensate for varying angles between the pole and the overhead wire, resulting from pivotal movement of the pole caused by varying heights of the overhead wire. The shoe is normally provided with a channeled contact which serves to keep the shoe in contact with the overhead wire despite limited movement of the bus thereunder. Specifically, such limited movement of the bus with respect to the overhead wires, and the channeled contact of the shoes with the overhead wires, will normally cause the poles to be pivotally moved without causing the shoes to be disengaged from the overhead wires.
Because the electric current must normally be passed through the sliding contact interface surfaces of the ball-and-socket swivel joint, the current conducting capacity of the harp is rather limited. To optimize the current conducting capacity, a shunt is normally provided, which extends through the socket portion of the swivel joint. The shunt is normally spring loaded, so that it will fit tightly between the swivel joint and a transmission conductor therebelow, which is provided to carry the electric current into the bus for purposes of operating the bus. Even with such a shunt, the current conducting capability of such prior art harps have been optimized at about 200 amps RMS continuously, and about 400 amps for periods of several minutes, without damage or overheating.
The newer articulated, transit buses that many transit authorities are now seeking, are essentially fifty percent larger and heavier, and in the case of electric, articulated buses, the current demand is also significantly greater, which may exceed 300 amps RMS continuously. As a result, the harp for articulated, electric buses had to be redesigned, because the prior art harps, as described above, do not have the current conducting capacity as required by such articulated, electric buses. The harps as now used on articulated, electric buses are essentially the same as those prior art harps as described above, except for the fact that they have been modified to include an external shunt, namely, a "by-pass" comprising a flexible conductor cable, one end of which is rigidly bolted to an extension of the socket portion of the swivel joint, with the other end rigidly bolted to the transmission conductor carrying the electric current into the bus. Such an external shunt does provide a solid current path, thereby eliminating the need to conduct current through any sliding contact interface surfaces of the swivel joint. While this has been a solution to the current demand requirements of articulated buses, it has not been a completely satisfactory solution. Specifically, the external shunt, or by-pass cable, as described above, is known to interfere with the swivel action of the ball-and-socket swivel joint. Even though efforts have been made to optimize the flexibility of the cable, and more than sufficient length is provided to span between the contact ends without the possibility of stretching the cable taught, the cable can and does interfere with the swivel action of the swivel joint, often causing the shoe to become disengaged from the electric overhead cable. Any such disengagement requires the bus operator to step outside the bus, and re-engage the harp with the overhead cable. Often times, the external shunt will tend to hold the harp perpendicular to the overhead wire, greatly complicating the bus operator's re-engagement efforts. In this regard, it should be realized that the bus operator cannot manually reach the harp, but must remotely effect such re-engagement, by manually manipulating a non-conductive cable extending downwardly from the pole. Accordingly, there is a need for a harp for an electric trolley bus, and particularly, an articulated, electric trolley bus, having increased current conducting capability which can be achieved without an external shunt that can interfere with the free swivel action of the swivel joint.